SLOW-ARRAY-WARNING

If you use ACL2 arrays you may sometimes see a slow array warning.
We here explain what that warning means and some likely ``mistakes''
it may signify.

The discussion in the documentation for arrays defines what we
mean by the semantic value of a name. As noted there, behind the
scenes ACL2 maintains the invariant that with some names there is
associated a pair consisting of an ACL2 array alist, called the
semantic value of the name, and an equivalent raw lisp array.
Access to ACL2 array elements, as in (aref1 name alist i), is
executed in constant time when the array alist is the semantic value
of the name, because we can just use the corresponding raw lisp
array to obtain the answer. Aset1 and compress1 modify the raw lisp
array appropriately to maintain the invariant.

If aref1 is called on a name and alist, and the alist is not the
then-current semantic value of the name, the correct result is
computed but it requires linear time because the alist must be
searched. When this happens, aref1 prints a slow array warning
message to the comment window. Aset1 behaves similarly because the
array it returns will cause the slow array warning every time it is
used.

From the purely logical perspective there is nothing ``wrong'' about
such use of arrays and it may be spurious to print a warning
message. But because arrays are generally used to achieve
efficiency, the slow array warning often means the user's
intentions are not being realized. Sometimes merely performance
expectations are not met; but the message may mean that the
functional behavior of the program is different than intended.

Here are some ``mistakes'' that might cause this behavior. In the
following we suppose the message was printed by aset1 about an array
named name. Suppose the alist supplied aset1 is alist.

(1) Compress1 was never called on name and alist. That is, perhaps
you created an alist that is an array1p and then proceeded to access
it with aref1 but never gave ACL2 the chance to create a raw lisp
array for it. After creating an alist that is intended for use as
an array, you must do (compress1 name alist) and pass the resulting
alist' as the array.

(2) Name is misspelled. Perhaps the array was compressed under the
name 'delta-1 but accessed under 'delta1?

(3) An aset1 was done to modify alist, producing a new array,
alist', but you subsequently used alist as an array. Inspect all
(aset1 name ...) occurrences and make sure that the alist modified
is never used subsequently (either in that function or any other).
It is good practice to adopt the following syntactic style. Suppose
the alist you are manipulating is the value of the local variable
alist. Suppose at some point in a function definition you wish to
modify alist with aset1. Then write

(let ((alist (aset1 name alist i val))) ...)

and make sure that the subsequent function body is entirely within
the scope of the let. Any uses of alist subsequently will refer
to the new alist and it is impossible to refer to the old alist.
Note that if you write

(4) A function which takes alist as an argument and modifies it with
aset1 fails to return the modified version. This is really the same
as (3) above, but focuses on function interfaces. If a function
takes an array alist as an argument and the function uses aset1 (or
a subfunction uses aset1, etc.), then the function probably
``ought'' to return the result produced by aset1. The reasoning
is as follows. If the array is passed into the function, then the
caller is holding the array. After the function modifies it, the
caller's version of the array is obsolete. If the caller is going
to make further use of the array, it must obtain the latest version,
i.e., that produced by the function.